The process of colour printing by photolithography involves the separation of the colours of the image into a number of components (usually four) to be reproduced by printing inks of corresponding colour (usually yellow, magenta, cyan and black).
Each colour separation is converted into the form of a halftone dot pattern by which tone rendition is achieved in lithographic printing. The perceived density of a particular colour on the final print depends on the relative size of the halftone dots in that area. It has recently become the practice to carry out both the colour separation and the generation of halftone dots automatically using a colour separation scanner of the electronic dot generation (EDG) type. The four halftone separation images are processed electronically and imaged separately onto black and white silver halide films using a scanning laser device. The printing plates are prepared from these four silver images of their duplicates by contact exposure. A further development in this area is the increasing use of electronic pagination systems which can manipulate the digitally stored image data for the purpose of page composition.
A very desirable adjunct to the electronic scanner and pagination systems is a method of producing a colour proof directly from the electronically stored data without the requirement for intermediate black and white images on silver halide film.
Several methods for the production of colour proofs directly from electronically stored images are known. It is possible to represent the image on a colour cathode ray tube which may be photographed using any of the commercially available colour photographic materials. Alternatively, a black and white cathode ray tube may be photographed sequentially through different spectral filters. A more sophisticated device which has become available enables the image to be scanned in continuous tone form onto conventional photographic colour paper using blue, green and red light from argon-ion and helium-neon lasers. An additional method is to use the signals to a colour TV monitor to drive a continuous tone scanning device which uses a white light source through red, green and blue filters, to expose a diffusion transfer material.
There are fundamental limitations to the usefulness of the known direct colour proofing methods. In particular, it is not possible to record the image in the exact form that it will finally appear, that is, as superimposed yellow, magenta, cyan and black images of halftone structure.
In one respect this limitation is imposed by the selection of photographic colour materials which are available. All of the silver halide colour recording materials presently available which work by the subtractive principle produce images which are formed from dyes of three colours: yellow, magenta and cyan.
It is recognised in the printing industry that a colour proof should be an exact representation of the final printed image produced from four superimposed halftone images in yellow, cyan, magenta and black inks. This is not readily feasible using known colour photographic materials because the yellow, magenta and cyan images have to be modified to compensate for the absence of a black layer. The result is, therefore, one stage removed from a genuine proof.
A further drawback of known methods employing conventional colour photographic materials is the limitation that the final images comprise a continuous tone form rather than the halftone form of the final printed image. Since one of the principal reasons for making a proof is to check whether the sizes of the yellow, magenta, cyan and black halftone dots are correct to produce the desired hue and tone, the proof should be composed of halftone dots rather than continuously varying density calculated to produce the same visual effect. The current use of continuous tone exposures is probably dictated by (1) the resolution of the imaging devices in use, (2) the extra equipment cost for computing equivalent yellow, magenta and cyan halftones to the yellow, magenta, cyan and black halftones, (3) the low to medium contrast of commercially available photographic colour materials which makes them not ideal for halftone exposures, and (4) the limited resolution of conventional chromogenic colour paper.
For these reasons the direct colour proofing methods presently available have not achieved widespread acceptance except as a check on page layout and composition. It is still common practice to produce high quality colour proofs either by actually printing on a special press or by laminating together individual yellow, magenta, cyan and black images formed in various ways by contact exposure through halftone separations on black and white film. These methods are generally time consuming and often require a high level of skill on the part of the operator.
Our copending British Patent Application No. GB 2172118A discloses a radiation-sensitive element suitable for colour proofing comprising a substrate bearing at least four separate imaging media coated thereon, said imaging media including:
(1) an imaging medium capable of forming a yellow image upon imagewise exposure and processing, PA1 (2) an imaging medium capable of forming a magenta image upon imagewise exposure and processing, PA1 (3) an imaging medium capable of forming a cyan image upon imagewise exposure and processing, and PA1 (4) an imaging medium capable of forming a black or a balancing black image upon imagewise exposure and processing, PA1 (1) an imaging medium capable of forming a yellow image upon imagewise exposure and processing, PA1 (2) an imaging medium capable of forming a magenta image upon imagewise exposure and processing, PA1 (3) an imaging medium capable of forming a cyan image upon imagewise exposure and processing, and PA1 (4) an imaging medium capable of forming a black image or balancing black image upon imagewise exposure and processing,
each imaging medium having a maximum spectral sensitivity at a wavelength different from that of the maximum sensitivity of the other imaging media.
The four layer elements are particularly suitable for the generation of highly accurate half-tone colour proofs. The element is exposed by four independent sources of different wavelengths and image formation in each layer is attributable only to a single source. Thus each layer may be truly representative of the printing plate used to apply the corresponding ink in the printing process.
These elements are based on an entirely different principle to conventional colour photographic silver halide elements. Conventional elements produce a colour image by combinations of cyan, magenta and yellow dyes and the wavelength of the exposing radiation causes image formation with a dye having the same wavelength within its principal absorption band. Thus a black image is formed by a combination of all three dyes generated by exposure of different wavelengths and there is no provision for generating black or balancing black by exposure to a single wavelength. The four layer elements utilize false-colour address in order to separate magenta, cyan, yellow and black. Thus the wavelength of the exposing source used to indicate a particular photosensitive layer is entirely independent of the colour generated in that layer. For example, a magenta separation may be digitised and thereafter be used with an infra-red emitting source to expose an imaging layer sensitive to infra-red. This material, on processing, generates a magenta image. Hitherto false-colour address has been used only for specialised image recording, e.g. infra-red aerial photography and X-ray photography and the elements used have not possessed the four layers of the elements of the invention.
The imaging media of the elements are selected such that not only does each medium have a maximum spectral sensitivity at a wavelength which is different from the wavelengths of maximum spectral sensitivity of the other imaging media, but each imaging medium has a sensitivity at the wavelengths of maximum spectral sensitivity of the other imaging media which is not significant so that upon image-wise exposure of the element to radiation of a wavelength corresponding to the maximum spectral sensitivity of one of said imaging media of sufficient intensity to cause image formation in that medium image formation will be confined to said one imaging medium. Thus, upon irradiation by four independent sources having wavelengths corresponding to the maximum spectral sensitivity of the layers and subsequent processing, the elements of the invention form super-imposed yellow, magenta, cyan and black or balancing black images, each image being attributable to the image-wise exposure of the respective source.
The elements can be utilised as a colour proofing system which can produce four colour halftone proofs of high accuracy directly from electronically processed halftone separation image data. The digitally processed images are used to modulate independent sources of actinic radiation, e.g. light emitting diodes (LED), laser diodes or infrared emitting diodes (IRED), which are selected to emit at the wavelength of maximum spectral sensitivity of the medium corresponding to the digitally processed image. The four independent exposures may be conducted simultaneously or sequentially since the spectral sensitivities of the imaging media are selected such that exposure from one source will cause latent image formation in one imaging medium without significantly affecting the other imaging media.
For colour proofing applications, in which the yellow, magenta and cyan colourants are matched in hue saturation and density to the printing inks, it follows that a fourth black layer is needed to provide adequate black density since this would not be achieved by a sum of the yellow, magenta and cyan alone. However, the sensitisation of all four layers to different spectral regions, whilst possible, does impose stringent conditions on the spectral sensitivity characteristics of each of the four colour forming layers and the spectral emission characteristics of the light sources. These conditions would be considerably relaxed if there were a method of sensitising the four layers to just three wavelengths.
It has now been found that it is possible to utilize a four layer (yellow, magenta, cyan and black or balancing black) element sensitised to only three different wavelengths to obtain high quality, half tone, full colour images suitable for use in a colour proofing system.